The 60 GHz band is an unlicensed band that features a large amount of bandwidth which means that a very high volume of information can be transmitted wirelessly. As a result, multiple applications that require transmission of a large amount of data can be developed to allow wireless communication around the 60 GHz band. Examples for such applications include, but are not limited to, wireless high definition TV (HDTV), wireless docking station, wireless Gigabit Ethernet, and many others. The objective of the industry is to integrate 60 GHz band applications with portable devices including, but not limited to, netbook computers, tablet computers, smartphones, laptop computers, and the like. The physical size of such devices is relatively small, thus the area for installing additional circuitry to support 60 GHz applications is limited.
Recently developed techniques allow fabricating integrated circuits (ICs) that operate in millimeter wave frequencies. An example for one such technique is the monolithic microwave integrated circuit (MMIC) or radio frequency IC (RFIC). The dimensions of RFICs are relatively small, ranging from around 1 square-millimeter (mm2) to 10 mm2 and such RFICs can be mass-produced.
However, even when designing a RF circuit as an IC, there still is a need to integrate many electronic elements in the same IC while keeping the size of the fabricated chip as small as possible and while not degrading performance of the IC. In addition, there is a need to fabricate such a design with minimum design constraints.
Phase shifters and hybrids are examples for electronic elements that implement a coupling function. Such elements are designed to achieve a specific phase difference and impedance matching between ports. A hybrid is a form of a reciprocal four-port device that provides a phase difference of 90-degrees or 180-degrees between two designated ports. A phase shifter can be implemented based on the hybrid to provide a controllable phase change of a RF signal as part of a phase array antenna.
One technique for designing of a hybrid element is based on a coupled transmission lines structure 100 as illustrated in FIG. 1. The length of the transmission lines 110 is typically λ/4. However, when fabricating the lines on a multilayer semiconductor substrate the length of each line is shortened due to the characteristics of the substrate, but still the length of each metal line 110 is not less than 0.6 mm (600 micron, instead of 1.25 mm). Mutual coupling is achieved when the metal lines 110 are perfectly adjacent to each other. The coupling may be degraded, hence the efficiency of the structure 100 due to increased line spacing, line impedance differences, and/or line lengths less than λ/4 is also degraded. Other conventional techniques for designing a hybrid are the branch-line coupler and the rat-race Hybrid, which typically include 4 transmission lines.
The coupled transmission lines structure 110 may be implemented in a form of a lumped element to achieve good coupling. A diagram illustrating such an implementation is shown in FIG. 2. In the lumped element 200, transmission lines are implemented as a LC (pi) network that includes inductors 210 and capacitors 220. The inductors 210 are wound on the substrate. However, such implementation requires specific inductors and capacitors, thus resulting in a larger area and more gain losses in comparison to the structure 100.
Thus, one of the drawbacks of the conventional coupled transmission lines structure and an equivalent lumped element is that they are considerably large in size for IC designs. Therefore, such structures are unsuitable for use in RFICs' designs, hence such structures are not partial in millimeter wave electrical elements, and more particularly elements that should be integrated in devices for 60 GHz applications.
Therefore, it would be advantageous to provide a solution for designing compact size millimeter wave electrical elements that are based on a coupling function.